It is well known to use extruded tubes of polytetrafluor-ethylene (PTFE) as implantable intraluminal prostheses, particularly vascular grafts. PTFE is particularly suitable as an implantable prosthesis as it exhibits superior biocompatibility. PTFE tubes may be used as vascular grafts in the replacement or repair of a blood vessel as PTFE exhibits low thrombogenicity. In vascular applications, the grafts are manufactured from expanded polytetrafluorethylene (ePTFE) tubes. These tubes have a microporous structure which allows natural tissue ingrowth and cell endothelization once implanted in the vascular system. This contributes to long term healing and patency of the graft.
Grafts formed of ePTFE have a fibrous state which is defined by interspaced nodes interconnected by elongated fibrils. The spaces between the node surfaces that is spanned by the fibrils is defined as the internodal distance (IND). A graft having a large IND enhances tissue ingrowth and cell endothelization as the graft is inherently more porous.
The art is replete with examples of microporous ePTFE tubes useful as vascular grafts. The porosity of an ePTFE vascular graft can be controlled by controlling the IND of the microporous structure of the tube. An increase in the IND within a given structure results in enhanced tissue ingrowth as well as cell endothelization along the inner surface thereof. However, such increase in the porosity of the tubular structure also results in reducing the overall radial tensile strength of the tube as well as reducing the ability for the graft to retain a suture placed therein during implantation. Also, such microporous tubular structures tend to exhibit low axial tear strength, so that a small tear or nick will tend to propagate along the length of the tube.
The art has seen attempts to increase the radial tensile and axial tear strength of microporous ePTFE tubes. These attempts seek to modify the structure of the extruded PTFE tubing during formation so that the resulting expanded tube has non-longitudinally aligned fibrils, thereby increasing both radial tensile strength as well as axial tear strength. U.S. Pat. No. 4,743,480 shows one attempt to reorient the fibrils of a resultant PTFE tube by modifying the extrusion process of the PTFE tube.
Other attempts to increase the radial tensile, as well as axial tear strength of a microporous ePTFE tube include forming the tubular graft of multiple layers placed over one another. Examples of multi-layered ePTFE tubular structures useful as implantable prostheses are shown in U.S. Pat. Nos. 4,816,338; 4,478,898 and 5,001,276. Other examples of multi-layered structures are shown in Japanese Patent Publication nos. 6-343,688 and 0-022,792.
While each of the above enumerated patents provides tubular graft structures exhibiting enhanced radial tensile strength, as well as enhanced axial tear strength, these structures all result in tubes exhibiting lower porosity. More specifically, the multi-layered ePTFE tubular structures of the prior art exhibit a smaller microporous structure overall, especially at the inner surface, and accordingly, a reduction in the ability of the graft to promote endothelization along the inner surface.
It is therefore desirable to provide a ePTFE vascular graft which exhibits increased porosity especially at the inner surface thereof while retaining a high degree of radial strength especially at the external surface thereof
It is further desirable to produce an ePTFE vascular graft which exhibits increased porosity at the outer surface thereof while retaining a high degree of radial tensile and suture retention strengths.